Binocular diplopia, otherwise known as double vision, is a visual symptom which may be brought about by any disorder which disturbs the motion of the eyes.
Two things are necessary for this symptom to emerge: first, a visual image must be perceived through each eye to a comparable (but not necessarily equal) extent; and second, the motion of the eyes must be disconjugate over at least part of the range of gaze. Any patient with normal vision in both eyes is therefore likely to develop binocular diplopia whenever a neural, muscular, or mechanical cause disturbs one or both eyeballs in a way that causes their motion to become disconjugate.
Binocular diplopia has two consequences for the patient: (a) it disrupts visual perception as a result of the interference between the two separate partly overlapping images that are perceived; and (b) it abolishes the component of the perception of depth (stereoscopic vision) that depends on the normal binocular disparity between the images in the two eyes. The effects of binocular diplopia can be particularly severe when there is relative motion between the patient and objects in the field of vision; the patient can experience dizziness and blurring of images in their field of view, and the brain's activity in trying to resolve these images can lead to headaches and mental fatigue.
Both consequences (a) and (b) above may be treated by restoring the conjugate motion of the eyes: either by removing the causative disturbance or by realigning the eyes with the aid of surgery. Alternatively, the two images may be brought into alignment with the aid of spectacle-mounted prisms that displace the image in one eye to compensate for the displacement of the eye itself.
Unfortunately, none of these treatments is universally applicable. The pathological cause is frequently irreversible; and surgery is possible only in some cases and even then only if and when the ocular motion disturbance is stable (generally at least 6 months' stability would first need to be observed). Similarly, prisms are only helpful where the deficit is fixed or changing at a rate substantially slower than that at which the patient can practically attend an orthoptic clinic; even then it often cannot cover the full range of gaze.
A great many patients therefore require an alternative approach (even if temporarily), namely, occlusion of the vision in one eye, usually the eye whose movement is (most) impaired. This is commonly done by applying a frosted plastic tape (or a clip-on occluder) on one of the two lenses of a pair of spectacles, or by fitting an eyepatch over the impaired eye, or, more rarely, by inserting a completely occlusive contact lens over the impaired eye. Since one image is abolished completely, the patient ceases to experience any perception of double vision; stereoscopic vision, however, remains lost. Understandably, patients dislike wearing eyepatches or clip-on occluders since they are visible to onlookers and may cause embarrassment.
Although very effective at abolishing diplopia, in addition to their cosmetic disadvantages, occlusive methods have a major flaw: they dramatically reduce the subject's visual field (to between 48% and 76% of the binocular visual field), necessarily so, since the vision in one eye is completely occluded. Patients are therefore less able to perceive items in the periphery of their vision on the occluded side, which can have a serious effect on their ability to interact with the immediate environment, potentially placing them at risk of accidents.
There is therefore a desire to treat binocular diplopia without substantially reducing the perception of items in the periphery of the field of vision of the occluded eye. It is estimated that about 25,000 patients annually in the UK alone would potentially benefit from better treatment in this situation.